It has been suggested that dietary intake of ascorbic acid in amounts well above the Recommended Dietary Allowance produces vitamin B12 deficiency in humans by destroying vitamin B12 during transport through the gastrointestinal tract and after absorption in blood and other tissues,as well. This investigation will study the reported effects of excess dietary ascorbic acid on vitamin B12 nutriture in the rat under controlled physiological conditions in order to determine whether there is any physiological or biochemical basis for a risk of developing vitamin B12 deficiency in humans. Groups of weanling rats will be fed diets with or without supplementary vitamin B12 and given ascorbic acid in their drinking water in order to duplicate the situation in man when vitamin C tablets are taken at mealtime. Vitamin B12 concentrations in the diet, tissues, and serum will be determined by microbiological assay with Lactobacillus leichmannii and urinary methylmalonic acid excretion will be measured as a specific diagnostic of vitamin B12 dificiency. Tissue and serum ascorbic acid will be determined by the dinitrophenylhydrazine method.